This invention relates generally to the retreading of tires. More particularly this invention relates to the retreading of tire casings with precured treads.
Traditionally a precured rubber tread is applied over the crown region of a tire casing in which a layer of vulcanizable rubber-based material or cushion gum is interposed between the tread and the tire casing. A flexible airtight member or envelope is then placed over the tread to cover at least a portion of the sidewalls of the tire casing. The envelope helps to provide pressure to the tread to enhance the bonding of the tread to the tire casing. It has been found that this method does not apply pressure uniformly over the entire bonding line. The envelope tends to bridge the groove areas causing a lower pressure to be applied at the base of the groove as opposed to the remainder of the tread. This unequal pressure can cause irregularities, such as reduction of nonskid, tread distortion, etc. Such problems have been set forth in U.S. Pat. No. 3,325,326 to Schelkmann.
In order to overcome these problems it was determined to apply fluid pressure to the envelope at a reduced pressure from that of the chamber curing temperature. Several such systems are disclosed in U.S. Pat. Nos. 3,325,326, 4,151,027 to Schelkmann et al. and 4,434,018 to Brewer. In each of these systems it is believed that by maintaining a pressure differential of about 15 psi between the pressure under the envelope and the chamber, the bridging effects of the envelope will be reduced. Specifically in U.S. Pat. No. 4,151,027 and 4,434,018 the 15 psi differential is maintained once the chamber pressure reaches 15 psi until the end of the cure cycle.
However, it has been found that during a cure cycle that the cushion gum does not soften, flow and cure uniformly across the bond line. As used herein, the bond line is the interface in cross-section between the tread and the tire casing in the axial direction. In other words the bond line extends from one axial tread edge to the other axial tread edge. This non-uniformity associated with the bond line can still result in tread distortion and reduced adhesion of the tread to the tire casing.
Another problem which can result from keeping a constant pressure differential between the chamber and the envelope is that too great of a pressure can be added before the tread edges are sealed, that is cured to the tire casing. This can result in infiltration of air, steam or other contaminants between the tread and the tire casing, thereby affecting the bond there between and/or the useful life of the treaded tire. For example, this can result when a leak in the envelope develops before the chamber pressure reaches its cure pressure.